Currently, in 3GPP (3rd Generation Partnership Project), the W-CDMA system has been standardized as a 3rd cellular mobile communication system, and its service has been started sequentially. Further, HSDPA (High Speed Downlink Packet Access) with the communication speed further increased has also been standardized, and its service is being started.
Meanwhile, in 3GPP, evolution in 3rd Generation Radio Access (Evolved Universal Terrestrial Radio Access: hereinafter, referred to as “EUTRA”) has been studied. As downlink in the EUTRA, an OFDM (Orthogonal Frequency Division Multiplexing) system is proposed. Further, proposed as uplink in the EUTRA is a DFT (Discrete Fourier Transform)—spread OFDM type single carrier communication system.
As shown in FIG. 14, the uplink of EUTRA is formed of an uplink pilot channel UPiCH, random access channel RACH, and uplink scheduling channel USCH (for example, see Non-Patent Document 1).
The uplink random access channel RACH of E-UTRA contains a non-synchronized random access channel and synchronized random access channel. Herein, a band of 1.25 MHz is used as a maximum unit of the non-synchronized random access channel. Then, for example, as shown in FIG. 15, a plurality of channels for access is prepared, and configured to be able to respond to a number of accesses.
Among intended purposes of the non-synchronized random access channel, it is the biggest purpose to synchronize a mobile station apparatus (hereinafter, referred as a “mobile station”) and base station apparatus (hereafter, referred to as a “base station”). Further, it is considered that several-bit information is transmitted to request scheduling for allocating radio resource, and the like to decrease the connection time between the mobile station and base station. Meanwhile, the intended purpose of the synchronized random access channel is to make a scheduling request (Non-patent Document 2).
On the non-synchronized random access channel, only a preamble is transmitted to acquire synchronization. This preamble contains a signature that is a signal pattern indicative of information, and by preparing a few tens of kinds of signatures, it is possible to designate several-bit information. Currently, it is anticipated that information of 4˜6 bits is transmitted, and that 16˜64 kinds of signatures are prepared. For example, expected as the information of 4˜6 bits are a reason of random access, downlink path-loss/CQI (Channel Quality Indicator), random ID and the like. Particularly, in the reason of random access, it is studied to designate handover, initial access, maintenance of synchronization, scheduling request or the like to make the access procedure efficient.
Herein, a configuration example of signatures included in the preamble is described with reference to FIGS. 16 and 17. FIG. 16 shows a configuration example of signatures in the case of splitting each kind of information to a field to designate the information. Shown herein is the case of allocating 2 bits to the reason of random access, 1 bit to random ID, and 1 bit to CQI. In the reason of random access, for example, “00” is selected in designating handover, while “11” is selected in designating maintenance of synchronization. Meanwhile, FIG. 17 shows the case of flexibly selecting the reason of random access, CQI and random ID to designate the information. The case is shown that codes from 0 to 15 are assigned to combinations of the reason of random access, CQI and random ID.
FIG. 18 is a sequence chart to explain an example of a conventional procedure of random access. FIG. 18 shows the procedure of random access in the case of using a non-synchronized random access channel. As shown in FIG. 18, in the conventional procedure of random access, a mobile station first selects a signature based on the reason of random access, downlink path-loss/CQI information, random ID and the like (step (hereinafter, abbreviated as “ST”) 1801). Then, the mobile station transmits a preamble (random access preamble) containing the selected signature on the non-synchronized random access channel (ST1802).
Upon receiving the preamble from the mobile station, the base station calculates a synchronization timing deviation between the mobile station and base station from the preamble, and performs scheduling for transmitting an L2/L3 (Layer2/Layer3) message (ST1803). Then, the base station assigns C-RNTI (Cell-Radio Network Temporary Identity) to the mobile station requiring C-RNTI from the random access reason, and transmits synchronization timing deviation information (synchronization information), scheduling information, signature ID number and C-RNTI (ST1804).
Upon receiving the information from the base station, the mobile station extracts a response from the base station including the transmitted signature ID number (ST1805). Then, the mobile station transmits an L2/L3 message with radio resources subjected to scheduling in the base station (ST1806). Upon receiving the L2/L3 message from the mobile station, the base station sends back a response corresponding to the message (ST1807).
A problem of such random access is that a collision occurs in the case that a plurality of different mobile stations selects the same signature and random access channel. When a plurality of mobile stations selects the same signature and transmits the signature with a radio resource block having the same time and frequency i.e. on the same random access channel, a collision occurs in the preamble (ST1802) as shown in FIG. 18.
When the base station cannot detect the preamble (ST1802) due to such a collision, the base station cannot send back the response (ST1804) including the synchronization information and the like. In this case, the mobile station cannot receive the response (ST1804) from the base station, and therefore, needs to select a signature and random access channel again after a lapse of predetermined time to perform random access.
Meanwhile, when the base station can detect the preamble (ST1802), the base station calculates L2/L3 message scheduling and synchronization timing deviation, and sends back a response (ST1804) to the mobile station. However, a plurality of mobile stations receives the response (ST1804) from the base station. Therefore, the plurality of mobile stations transmits the L2/L3 message (ST1806) with radio resources subjected to scheduling, and as a result, the collision occurs in the L2/L3 message (ST1806).
When the base station cannot detect the L2/L3 message (ST1806) due to such a collision, the base station cannot send back the response (ST1807). In this case, the mobile station cannot receive the response (ST1807) from the base station, and therefore, needs to select a signature and random access channel again after a lapse of predetermined time to perform random access. Thus, when a plurality of mobile stations selects the same signature and random access channel, the collision can occur, while when the collision occurs, the time up to ST1807 as shown in FIG. 18 is required at the maximum until the collision is detected.
Meanwhile, when a mobile station capable of executing random access is located in a position as shown in FIG. 19, handover is executed. Also when handover is executed, the above-mentioned random access is performed.
Described herein is an example of a procedure of random access at the time of executing handover. FIG. 20 is a sequence chart to explain an example of a procedure of random access at the time of executing handover. In addition, as in FIG. 18, FIG. 20 shows the procedure of random access in the case of using a non-synchronized random access channel.
As shown in FIG. 20, in the procedure of random access at the time of executing handover, as a preparatory stage, a mobile station first measures radio conditions of adjacent base stations (ST2001). Then, the mobile station transmits the measurement result (measurement report) to base station A that is a base station (hereinafter, “local-base station” as appropriate) currently holding the mobile station (ST2002). Upon receiving the measurement result from the mobile station, the base station A selects an optimal base station from the measurement result (ST2003). In addition, herein, base station B is assumed to be selected as an optimal base station. Then, the base station A transmits a handover request command to the base station B that is a handover destination (ST2004).
Upon receiving the handover request command from the base station A, the base station B assigns C-RNTI to the mobile station performing handover (ST2005). Then, as a response to the handover request, the base station B notifies the base station A of a handover request acknowledge command including the C-RNTI (ST2006). Upon receiving the handover request acknowledge command from the base station B, the base station A transmits a handover command including the C-RNTI to the mobile station (ST2007).
Upon receiving the handover command from the base station A, the mobile station acquires synchronization on downlink of the base station B, and confirms a position of the random access channel from the broadcast channel (ST2008). When the downlink synchronization is acquired, the mobile station selects one signature from among signatures such that the reason of random access is handover (ST2009). Then, the mobile station transmits a preamble (random access preamble) containing the selected signature to the base station B on the random access channel (ST2010).
Upon detecting the signature from the preamble received from the mobile station, the base station B calculates a synchronization timing deviation, and performs scheduling of uplink for the mobile station to transmit a handover completion message (ST2011). Then, the base station B transmits synchronization timing deviation information (synchronization information), scheduling information and signature ID number (ST2012). In addition, in the case that the random access reason is handover, C-RNTI is beforehand notified, and therefore, is not transmitted.
Upon receiving the information to the mobile station from the base station B, the mobile station corrects the synchronization timing deviation based on the synchronization timing deviation information (synchronization information) (ST2013). Then, the mobile station transmits a handover completion message with radio resources subjected to scheduling (ST2014). Upon receiving the handover completion message from the mobile station, the base station sends back a response corresponding to the message (ST2015).    Non-patent Document 1: R1-050850 “Physical Channel and Multiplexing in Evolved UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #42 London, UK, Aug. 29-Sep. 2, 2005    Non-patent Document 2: 3GPP TR (Technical Report) 25.814, V7.0.0(2006-June), Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)